$a=-i\sqrt{x}$
$a=i\sqrt{x}$
$a=\sqrt{x}$
$a=-\sqrt{x}$

$\left\{\begin{matrix}a=\sqrt{-x}\text{; }a=-\sqrt{-x}\text{, }&x\leq 0\\a=\sqrt{x}\text{; }a=-\sqrt{x}\text{, }&x\geq 0\end{matrix}\right.$

$x=a^{2}$
$x=-a^{2}$